The present invention relates generally to wireless communication systems. More particularly, the present invention relates to stabilizing a network to minimize delays experienced by users when trying to access the network.
In wireless communication systems, the electromagnetic spectrum that comprises the channels available to a network operator is a precious commodity. In most systems the number of subscribers far exceeds the number of available channels, thus means for providing network resources on demand must be provided. In digital cellular telephone systems this xe2x80x9crandom accessxe2x80x9d problem is solved by means of slotted Aloha. Aloha is a well-known term of art defining xe2x80x9chandshakingxe2x80x9d or allotting and acknowledging a communications connection path. In the unslotted and slotted Aloha protocols, users transmit on a common channel. If two or more users transmit at substantially the same time, they are said to xe2x80x9ccollidexe2x80x9dxe2x80x94in many cases this means that neither user is successful in getting through to the network. In slotted ALOHA, the users are constrained to transmit in time slots, thus user transmissions completely overlap and collide or do not overlap at all. In unslotted ALOHA, a user transmits whenever he or she wishes and there is a time window during which a transmission by another user causes a collision. The colliding users must retransmit at later, randomly selected, times to reduce the risk of further collisions. It is important to note that the retransmission timing must be random to reduce the risk of further collisions. Users that have suffered a collision and are waiting to retransmit are said to be xe2x80x9cbackloggedxe2x80x9d users.
In digital cellular systems, a separate Aloha channel is provided for registration and channel access. Registration is the process by which a mobile informs a base station of the mobile""s presence and ability to place and receive calls. FIG. 1 of the drawings shows a known, prior art simplified protocol for channel access and voice/data wireless transmission. xe2x80x9cForwardxe2x80x9d transmissions are defined as those between the Base Station Controller (BSC) and the mobile user. xe2x80x9cReversexe2x80x9d transmissions are those from the mobile to the BSC. Forward and reverse transmissions may use distinct frequencies (frequency division duplexedxe2x80x94FDD). Forward and reverse transmissions may also be on the same channel (time division duplexedxe2x80x94Txcex94xcex94). In the example in FIG. 1, however, both the forward and reverse channels are time division multiplexed to accommodate multiple users (time division multiple accessxe2x80x94TDMA). An Aloha segment is included on the reverse channel to allow for user registration and requests for access.
FIG. 1 also shows the timing of a simple request for reverse channel time. The mobile user sends a request to set up a call on the Aloha portion of the reverse channel. The network responds with a grant of access on the forward channel. This grant of access allocates a particular pair of channels to the mobile user over which the call can proceed. Aloha protocols thus provide a simple and cost effective means for controlling access to the network resources.
It is therefore an object of the present invention to stabilize slotted Aloha by managing channel loading.
It is another object of the present invention to provide a simple technique for determining the number of backlogged users.
It is yet another object of the present invention to provide and set parameters for a backoff algorithm to accommodate colliding callers.
The present invention meets the foregoing objects in systems and methods for designing a system to control the probability qr with which backlogged users retransmit. Advantage is made of the fact that if the number of backlogged users is known, the retransmission probability can be varied appropriately. The present invention provides a technique for determining the number of backlogged users which is used to set parameters to a backoff algorithm to better service and accommodate colliding callers.
In this invention a collection of sensors is used to collect energy measurements throughout the network coverage area. The sensor data is processed by an artificial neural network that has been trained to determine the numbers of colliding users, and also to determine whether a given transmitter (successful or not) was a member of an earlier group of colliding callers. This information is used to track the number of backlogged users, and to set the parameters of the backoff algorithm accordingly.
The sensor information can also be used to improve performance in those cases in which the number of active users exceeds the capacity of the channel. The sensors provide azimuthal information that is used to partition the Aloha channel into a number of subchannels whose collective capacity is sufficient to resolve contention among the backlogged users. The basic idea of the present invention is to use the sensors to determine the angle from which the transmissions are arriving at the base station. This allows the use of steerable beam antennas to separate colliding users that do not lie along the radial line moving out from the base station. This sectorization of the Aloha channel spatially resolves a large group of callers into smaller clusters of callers that can then be allowed access to the network in time through the backoff algorithm, thus achieving a higher throughput than that possible with a single Aloha channel.